1. Field of the Invention
The invention relates to a self-lubricating solid composite comprising chromium oxide dispersed in a metal binder. More particularly the invention relates to a friction reducing and wear resistant solid lubricant composite comprising Cr.sub.2 O.sub.3 in a metal binder, along with metal fluoride and, optionally, a metal effective as a lubricant at low temperature, for use over a wide temperature spectrum from cryogenic (-185.degree. C.) temperature to about 900.degree. C.
2. Background of the Disclosure
There is an increasing need for durable solid lubricant materials which will operate over wide range of temperatures and at high speeds, for long periods of time. Such materials find increasing need in space satellites and vehicles, adiabatic diesel and advanced turbo machinery, process control valve stems, dry running stirling engine cylinders, high speed foil air bearings, rotating face valves, butterfly valve stems, and the like. Numerous advances in the art have been made over the last thirty years since early self-lubricating compositions, such as composites of silver, platinum, molybdenum disulfide, lead oxide and silicon dioxide were disclosed in U.S. Pat. No. 3,199,934. A significant advance was made by Sliney as a porous nickel-chromium alloy in which is dispersed, via infiltration, a metal fluoride eutectic and, optionally, a coating of the eutectic and silver on the outer bearing surface as disclosed in U.S. Pat. No. 3,419,363. However, the porous metal provided a greatly increased surface area and high temperature oxidation of these porous sintered metals posed significant problems at temperatures above about 700.degree. C. This lead to the development of another self-lubricating composite in which the metal component is a porous high temperature alloy body which is either infiltrated with both metal fluorides and glass or via plasma spray codeposition of the component powders, as disclosed in U.S. Pat. No. 4,214,905. The presence of the glass in the composite increased the oxidation resistance of the metal binder. The silver is electrodeposited on the metal. A still further improvement in the art was the development of a more wear resistant composite of nickel-cobalt bonded chromium carbide with metal fluoride and silver which is known as PS/PM200 and is disclosed in U.S. Pat. No. 4,728,448 and which has been extensively published in the literature. While this material has met with much success as a durable, long lasting, wear resistant self-lubricating composite useful over a wide temperature range, it is expensive and the chromium carbide component is so hard as to require costly diamond grinding achieve the desired dimensions prior to service. Further, at very high temperatures of 800.degree. C. or more in an oxidative environment such as air, the chromium carbide tends to oxidize. This degrades the friction and wear properties and causes slight dimensional swelling of the composite body. It would be a significant improvement to the art if a material were available with the strength, low friction and wear characteristics of the PS/PM200, without the drawbacks of very high temperature oxidative instability, high component cost and the need for expensive diamond grinding to polish the bearing surfaces and achieve the proper dimensions of the composite body.